Coder-decoders are used in telecommunications equipment for diverse purposes. Typically, a coder-decoder refers to an analog to digital converter and a digital to analog converter that are contained within the same circuit, such that two-way data communications may be performed between analog systems and digital systems. The coder-decoder thus "codes" analog information into a digital format, and "decodes" digital information into an analog format.
Coder-decoders may be used in conjunction with amplification when the signal received at the coder-decoder input would otherwise be too weak to be correctly decoded. External amplification devices may be utilized for such purposes, but these devices typically drive up the cost of the overall system that utilizes the amplifier and the coder decoder. They must be separately manufactured and installed, which creates additional costs and decreases overall system reliability. For this reason, coder-decoders with internal amplification systems have been developed.
Nevertheless, such coder-decoders with internal amplification systems have drawbacks that limit the usefulness of the coder-decoder. If a large amplification gain is used, the amplification system may be easily driven into saturation. If a small amplification gain is used, then it may still be necessary to have additional external amplification installed. For these reasons, it is often necessary to design the amplification for each coder-decoder to meet the needs of the system that it is being used in.
For certain applications, this constraint for coder-decoders can result in non-optimal configurations. For example, it is often desirable to obtain signals from a telephone line, such as caller identification signals, while the line is on-hook. An AC bypass path may be provided by connecting a capacitor across the hook relay. Known coder-decoders require a large capacitance to be used, because otherwise, signal attenuation will result in a caller identification signal that is too weak to be used. If the signal is amplified, then the data channel signal (carrying voice data, digitally-encoded data, or other suitable communications data that may be transmitted when the receiver is off-hook) that is received when the line is off-hook will saturate the amplifier. This configuration is undesirable, as it results in a hook impedance that does not meet industry standards.